Concrete Reinforcement Calculations

Reinforcement Types

• Most steel reinforcing bar, called rebar, is “deformed," a term for the pattern of bumps on the surface that help concrete grip the bar. Smooth bars are used when they are intended to slide, such as expansion joints in concrete highway pavement. Smaller-sized rebar, both deformed and smooth, is sometimes made into reinforcing mesh or nets. Steel fibers added to concrete help control drying shrinkage to increase the bending strength of concrete. Synthetic fibers are sometimes added to concrete to prevent shrinkage, but they do not make it stronger. Steel plates can be bolted or glued to concrete for external reinforcement. This type of reinforcement is often used to strengthen existing concrete columns against earthquakes.

Reinforcement Basics

• Rebar is necessary in structural members such as beams that carry loads. Compression pushes down from the top of a horizontal beam while the bottom is subject to tension that stretches it. Reinforcement is needed for concrete beams, joists, girders, columns, domes and arches that must resist loads that produce tension. If reinforcement is not placed properly, the concrete can collapse. To increase the concrete’s resistance to tensile force, the rebar is sometimes bent, hooked or lapped. Weight on a concrete slab on the ground puts compression on the top and tension on the bottom. Rebar increases its tensile strength and helps prevent shrinkage cracks from widening. A simple concrete curb that does not bear weight does not need reinforcement.

Installation

• The rebar should be free of dirt, grease or rust. Allow enough room between the rebars so the concrete can compact around them. Compaction creates strong bonds between the rebar and the concrete. This helps prevent cracks from widening or developing at joints and helps the rebar accept increased tensile forces.

Rebar Size

• Rebar is usually sold in 20 to 60 foot lengths. ASTM International, formerly the American Standards and Testing Materials, sets the size and weight of rebar internationally. Size 3 is 0.375 inches wide and weighs 0.376 lbs. per foot (ppf). Size 4 is 0.50 inches wide and weighs 0.668 ppf. Size 5 is 0.625 inches wide and weighs 1.403 ppf. Size 6 is 0.750 inches wide and weighs 1.502 ppf. Size 7 is 0.875 inches wide and weighs 2.044 ppf. Size 8 is 1 inches wide and weighs 2.670 ppf. Size 9 is 1.128 inches wide and weighs 3.40 ppf. Size 10 is 1.270 inches wide and weighs 4.303 ppf. Size 11 is 1.40 inches wide and weighs 5.213 ppf. Size 14 is 1.693 inches wide and weighs 7.650 ppf. Size 18 is 2.257 inches wide and weighs 13.60 ppf.